HI, I seem to have the same issue, but my setup is a bit different.
I have OF 1.10.6 running on two Macs (one ElCap, and one Sierra), and syncing w two iOS devices running OF 2.19.1. Although OF1 is buggy in Sierra, the Syncing has been working fine for years.
I synced all devices, moved old data to archive in ElCap, and synced all again, as I do every month. Both of my Macs are now displaying “Unable to open zip archive.” error, although seems to be working fine in iOS. (On one Mac, I tried reverting to a backup, but same when syncs w new dB on server.)
How can I move all data to a new dB, compatible w Mac v1. I could resort to yesterday’s backup on Mac… but is there a way to see recently modified items? Any other way to solve this issue… and how did this happen, I’ve not done anything out of the ordinary.